diff --git a/src/benchmark_radar/data_store.py b/src/benchmark_radar/data_store.py
index f31b6017..bacb1e20 100644
--- a/src/benchmark_radar/data_store.py
+++ b/src/benchmark_radar/data_store.py
@@ -73,7 +73,7 @@ def _read_json(path: Path, *, label: str) -> dict[str, Any]:
             code="not_initialized",
             status=409,
         ) from error
-    except (OSError, json.JSONDecodeError) as error:
+    except (OSError, UnicodeDecodeError, json.JSONDecodeError) as error:
         raise DataSyncError(
             f"cannot read {label} at {path}: {type(error).__name__}: {error}",
             code="invalid_local_state",
diff --git a/tests/test_data_sync.py b/tests/test_data_sync.py
index c17a6bf7..dfc72347 100644
--- a/tests/test_data_sync.py
+++ b/tests/test_data_sync.py
@@ -546,3 +546,28 @@ def test_unsafe_archive_member_is_rejected(tmp_path: Path) -> None:
     with pytest.raises(DataSyncError, match="unsafe archive path"):
         store.initialize()
     assert not (tmp_path / "escaped.json").exists()
+
+
+def test_invalid_state_encoding_has_the_local_state_error_contract(tmp_path):
+    # A damaged UTF-8 state file bypassed DataSyncError, so offline clients
+    # received a traceback instead of the same contract as malformed JSON.
+    original = b'{"schema_version": 1, "data_version": "\xff"}'
+    (tmp_path / "state.json").write_bytes(original)
+    with pytest.raises(DataSyncError) as captured:
+        DataStore(root=tmp_path).state()
+    assert captured.value.code == "invalid_local_state"
+    assert (tmp_path / "state.json").read_bytes() == original
+
+
+@pytest.mark.parametrize("command", [["sync"], ["search", "agent"]])
+def test_corrupted_state_stays_structured_in_installed_cli(tmp_path, monkeypatch, capsys, command):
+    original = b"\xff"
+    (tmp_path / "state.json").write_bytes(original)
+    monkeypatch.setenv("BENCHMARK_RADAR_HOME", str(tmp_path))
+    assert run_query_cli([*command, "--json"]) == 1
+    output = capsys.readouterr()
+    payload = json.loads(output.err)
+    assert payload["error"]["code"] == "invalid_local_state"
+    assert output.out == ""
+    assert "Traceback" not in output.err
+    assert (tmp_path / "state.json").read_bytes() == original